1. Field of the Invention
The present invention relates to methods for controlling the composition and deposition rate of an electroless plating solution and to control apparatus therefor.
2. Background Art
Electroless plating solutions including electroless copper solutions are used in a wide range of industrial applications. In particular, electronic printed circuit boards are often manufactured by electroless deposition of copper on the board's circuit traces and through holes walls.
A typical electroless copper solution comprises a number of components including a cupric salt, a chelating or complexing agent such as ethylenediaminetetracetic acid (EDTA), a reducing agent such as formaldehyde and a caustic pH adjustor such as sodium hydroxide. Proprietary additives also are typically employed in relatively low concentrations to enhance various characteristics of the copper plate. Additionally, if the surface to be plated is not catalytic for electroless deposition, the substrate surface is treated with a catalyst prior to contact with the electroless plating solution. A suitable catalyst for electroless deposition is a palladium tin colloid catalyst, as disclosed for example in U.S. Pat. Nos. 4,061,588 and 3,011,920, incorporated herein by reference.
It is recognized that to provide a quality electroless deposit, the rate of metal deposition on a substrate should be maintained substantially constant. It is known that various plating bath components are continuously depleted during use of a plating solution, for example by plate-out or chemical reaction, and that concentration variations of bath components can vary the metal deposition rate.
Prior approaches for controlling the rate of copper deposition have included monitoring and controlling temperature, pH and chemical composition of the plating solution. For example, one approach provides for manual analysis of the plating solution during use of same followed by manual addition of plating bath components as shown to be necessary by analysis. Such manual analysis is often performed using "rate coupons". At intervals of approximately one hour, a determination of plating rate is made by measuring the weight gain of a copper coupon that has been suspended in the plating solution for said interval. The weight gain measurement on the copper coupon serves as a basis to add additional component(s), e.g. formaldehyde, to the plating bath and thereby control the copper deposition rate. A notable shortcoming of a rate coupon analysis is that it yields little information about the current plating rate. Consequently, the plating rate of a bath may vary widely and for extended periods before any attempts are made to return the plating rate to a desired level. It thus would be desirable to have a means to continuously and accurately monitor and maintain the deposition rate of a plating solution.
Relatively recently, a technique based on the use of an oscillating quartz crystal has been reported for use in determining mass change. This mass measurement method is often referred to as a quartz crystal microbalance or QCM. See R. Schumacher, Angewendte Chemie Int. Ed. Engl., vol. 29, no. 4, 329-343 (1990); W. Hinsberg, et al., Rev. Sci. Instrum., 60 (3), 489-492 (Mar. 1989), both incorporated herein by reference for their teaching of quartz microbalances and use of the same.